The aim of this thesis is the design of a subsonic turbine with high inlet Mach number. To improve the efficiency of gas turbines, recently it was investigated the possibility to replace a classic combustor with a Rotating Detonation Combustor (RDC). This is a novel technology based on the detonation combustion mode and thermodynamic cycle studies have shown optimistic results in terms of performance, efficiency and emission of pollutants. The flow coming out from the detonative combustor is supersonic, non-uniform and unsteady. Efficient design of supersonic turbines is possible to deal with the outlet supersonic flow, but losses associated to shocks will always be present. For this reason, we investigated an alternative approach to couple an RDC with a gas turbine, adopting a transition duct that decelerate the flow to a subsonic condition. Due to the high non uniformities and unsteadiness of the flow, the deceleration to conventional turbine inlet Mach number of 0.2÷0.3 will produce a high amount of losses, cancelling the advantage of detonative combustion. The design of the turbine was approached starting from a mean-line parametric analysis, to find the point of maximum efficiency. The mean-line software zTurbo was used and a tailored algorithm was created to run the analysis. Due to the particular inlet flow, we were out of validity range of losses correlations implemented in zTurbo, so an extrapolation strategy for Traupel correlations was adopted. It works well for profile losses, but it is inadequate for secondary ones. The following step was the design of the blade profile and meridional channel geometry through 2D CFD simulations. For both stator and rotor we varied the solidity, chord length, blade thickness, camber line and meridional channel shapes to find the combination that produces the lowest amount of losses. Then, a shape optimization procedure using software FORMA was performed to improve turbine performances. The optimized shapes were developed in 3D to assess the 3D flow behavior, finding optimistic results. Finally, mechanical assessment and off-design working conditions analysis were performed, showing that the blades can withstand the static loadings and that performance reduction in off-design conditions are limited.
Lo scopo di questo lavoro è la progettazione di una turbina subsonica con un flusso ad alto Mach in ingresso. Per migliorare l’efficienza delle turbine a gas, recenti ricerche hanno studiato la possibilità di adottare un Combustore a Detonazione Rotante (RDC), al posto del classico combustore. Si tratta di una tecnologia basata sulla combustione a detonazione e studi sul ciclo termodinamico hanno mostrato risultati ottimistici in termini di prestazioni, efficienza e riduzione delle emissioni inquinanti. Il flusso che esce dal combustore detonante è supersonico, non uniforme e instabile. Design efficienti di turbine supersoniche sono possibili, ma le perdite dovute agli shock sono sempre presenti. Per questo motivo abbiamo investigato un approccio alternativo per accoppiare RDC e turbina: creare un condotto di transizione che decelera il flusso rendendolo subsonico. A causa delle disuniformità e instabilità, la decelerazione fino a Mach convenzionali di 0.2÷0.3 produrrebbe tante perdite da vanificare i vantaggi della combustione a detonazione. La progettazione della turbina è partita da un’analisi parametrica alla linea media, per trovare il punto di massima efficienza. È stato utilizzato il software zTurbo per i calcoli alla linea media ed è stato creato un algoritmo specifico per l’analisi parametrica. A causa del flusso particolare, il caso analizzato è al di fuori del range di validità delle correlaioni delle perdite implementate in zTurbo, per cui abbiamo creato una strategia di estrapolazione per le perdite di Traupel. Questa funziona bene per le perdite di profilo, ma non funziona per quelle secondarie. Il passo successivo è stato la progettazione del profilo della pala e del canale meridionale tramite simulazioni CFD 2D. Per statore e rotore sono stati analizzati il numero di pale, la lunghezza della corda, lo spessore della pala e la forma della linea media e del canale meridionale per trovare la combinazione che produce meno perdite. Poi è stata eseguita un’ottimizzazione dei profili tramite il codice FORMA, per migliorare le prestazioni. I profili ottimizzati sono stati sviluppati in 3D per verificarne il comportamento, ottenendo risultati ottimisitici. Infine sono state eseguite due verifiche: meccanica e lavoro in condizioni off-design. Entrambe hanno restituito buoni risultati.
Preliminary design and optimization of a subsonic turbine for rotating detonation engine applications
BERTELLI, MATTEO
2021/2022
Abstract
The aim of this thesis is the design of a subsonic turbine with high inlet Mach number. To improve the efficiency of gas turbines, recently it was investigated the possibility to replace a classic combustor with a Rotating Detonation Combustor (RDC). This is a novel technology based on the detonation combustion mode and thermodynamic cycle studies have shown optimistic results in terms of performance, efficiency and emission of pollutants. The flow coming out from the detonative combustor is supersonic, non-uniform and unsteady. Efficient design of supersonic turbines is possible to deal with the outlet supersonic flow, but losses associated to shocks will always be present. For this reason, we investigated an alternative approach to couple an RDC with a gas turbine, adopting a transition duct that decelerate the flow to a subsonic condition. Due to the high non uniformities and unsteadiness of the flow, the deceleration to conventional turbine inlet Mach number of 0.2÷0.3 will produce a high amount of losses, cancelling the advantage of detonative combustion. The design of the turbine was approached starting from a mean-line parametric analysis, to find the point of maximum efficiency. The mean-line software zTurbo was used and a tailored algorithm was created to run the analysis. Due to the particular inlet flow, we were out of validity range of losses correlations implemented in zTurbo, so an extrapolation strategy for Traupel correlations was adopted. It works well for profile losses, but it is inadequate for secondary ones. The following step was the design of the blade profile and meridional channel geometry through 2D CFD simulations. For both stator and rotor we varied the solidity, chord length, blade thickness, camber line and meridional channel shapes to find the combination that produces the lowest amount of losses. Then, a shape optimization procedure using software FORMA was performed to improve turbine performances. The optimized shapes were developed in 3D to assess the 3D flow behavior, finding optimistic results. Finally, mechanical assessment and off-design working conditions analysis were performed, showing that the blades can withstand the static loadings and that performance reduction in off-design conditions are limited.File | Dimensione | Formato | |
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2022_07_Bertelli_01.pdf
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2022_07_Bertelli_02.pdf
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https://hdl.handle.net/10589/190207